The invention relates to a method of treating liquid including foreign substances, particularly municipal and commercial waste water, by means of a membrane separating apparatus including membrane elements wherein the liquid to be treated is admitted, by way of an inlet, to the admission side of the membrane apparatus and the retent with increased foreign substance concentration and the retent with increased foreign substance concentration and the permeant free of the foreign substances are conducted out of the apparatus through different discharge passages.
Such apparatus have been known for some time and are used in various separation processes for liquids including several components and also for gases. An important aspect in the separation of municipal and commercial waste waters by means of the type of apparatus described above is that the concentration of the substances to be removed can be increased in the retent to such a degree that they are present in a concentration which is optimal for the subsequent biological digestion process. To this end, it is necessary to retrieve the liquid component, that is, the water, from the medium to be separated in a single processing step with such a purity that it can be discharged as clean water suitable for other uses without further treatment. Another requirement for the operation of this type of an apparatus is that it uses relatively only a small amount of energy thereby providing for a good cost-utilization ratio.
The most recent state of the art apparatus of this type provide for satisfactory results with respect to the parameters given above. They are however subject to clogging and blockages during extended operation. The so-called cross-flow pipe systems which have a relatively high specific energy consumption are operating reliably even with a high content of solids in the raw water and with a high throughput, but systems with low specific energy requirements can operate only with relatively low throughput when the content of solids is relatively high. Particularly apparatus with high membrane densities exhibit a performance loss already with relatively low concentrations of solids in the raw water and they may completely fail inspite of periodic flushing of the membrane elements. As a result, the apparatus need to be disassembled frequently and tediously cleaned and re-assembled and installed into the cleaning system. The time intervals in which such cleaning procedures are required are different depending on the substance content of the medium to be separated and the type of substances, but in any case, they are too short for an effective and efficient cleanup operation of residential and commercial waste waters using apparatus of the above type known in the art.
The reason herefor is not only that the membrane elements are extremely vulnerable points of such apparatus, but also the way the media to be separated are conducted along and past the membranes generate problems. For such separation procedures, state of the art apparatus have been used wherein the membrane elements are disposed in a pressure-tight housing in the form of flat or plate-like elements and the medium to be separated is conducted through the housing of the apparatus along the membrane elements in a meander-like fashion from an inlet to a retent outlet. Also, the medium may flow past all the membrane elements in a parallel flow arrangement. The retent, which is enriched with the foreign substances leaves the apparatus and also the permeant, for example, clean water, leaves the apparatus housing by way of another, centrally arranged, outlet. Also, combinations of such apparatus are known in the art. For example, apparatus wherein part of the membrane elements are so arranged that the medium flows along the membrane walls wherein blocks of such parallel flow membrane elements are arranged in series.
All these apparatus have a disadvantage in that, because the flow through the apparatus must be redirected, they generate a relatively high hydrodynamic resistance. As a result, the medium must be admitted with a relatively high pressure in order to be able to flow past all the membrane elements. With the resulting relatively low flow speed of the medium to be separated the membrane elements can therefore easily be blocked by the foreign substances suspended in the medium.
It is the object of the present invention to provide an apparatus of the type referred to above which operates reliably and without blockages which would detrimentally affect operation, which can be easily disassembled and which is simple in its design and can be inexpensively operated.